Final answer:
The assassination of the Austrian Archduke Franz Ferdinand by Gavrilo Princip, a Serbian nationalist, on June 28, 1914, triggered the start of World War I. The event rapidly led to a series of declarations of war due to existing alliances, escalating into the first global and fully industrialized conflict with unprecedented casualties.
Step-by-step explanation:
The Assassination of Archduke Franz Ferdinand: The Spark That Ignited World War I
The event that started World War I was the assassination of the Austrian Archduke Franz Ferdinand on June 28, 1914. As the heir to the Austro-Hungarian throne, Franz Ferdinand was a significant figure in European politics. His death at the hands of Gavrilo Princip, a Serbian nationalist, set off a catastrophic chain of events that led to the outbreak of the Great War. At the time of his assassination, Franz Ferdinand had substantial control over the Austrian military and was known for his preference for peaceful resolution, suggesting that his survival might have led to a different historical outcome.
The first few months of World War I saw a rapidly escalating series of alliances and declarations of war. Following the assassination, Austria-Hungary issued an ultimatum to Serbia, which Serbia partially rejected. Austria-Hungary, backed by Germany, declared war on Serbia, and within weeks, a network of existing alliances drew in Russia, France, Great Britain, and eventually the rest of the world. The scale of the war was unprecedented, involving nations from nearly every continent. The combination of long-standing international tensions, a complex alliance system, and the industrialization of war led to a conflict of extraordinary destruction and a high number of casualties.
The assassination not only triggered the war but also marked a turning point in how modern warfare would be conducted. The conflict quickly escalated into trench warfare, with both sides employing new military technologies such as machine guns, mustard gas, and long-range artillery, marking World War I as the first fully-industrialized war in history.